Note: Descriptions are shown in the official language in which they were submitted.
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A stabilised antenna arra_~ement
This invention relates to stabilised antenna arrange-
ments and is particularly applicable to antennas which are
used aboard ships. It is often very important to ensure
that the pointing direction of the antenna is not adversely
affected by the pitching and rolling motion of the vessel,
since this can seriously reduce the sensitivity o~ the
antenna to weak, directional received signals. The problem
is particularly severe with marine surveillance radars,
which are required to rotate in azimuth so as to cover the
entire field of view surrounding the ship on which it is
mounted. Pitching and rolling motion of the ship will cause
errors, which will result in the direction of a target
identified by the radar being incorrectly indicated.
Various proposals have been put forward for stabilising
a ship borne antenna in azimuth, but these can be excessively
complex or can result in a very high centre of gravity. For
example, it has been proposed to mount an antenna on a
stabilised platform with the plat~orm being stabilised
against pitch and roll motions, and with the antenna being
rotatable in azimuth with respect to the platform.
The present invention seeks to provide an improved
stabilised antenna. In accordance with an aspect of the
invention there is provided a stabilised ship borne antenna
arrangement including a rotatable member arranged to rotate
about an unstabilised axis which, in use, is fixed relative
to the ship, a directional antenna mounted on the rotatable
member so as to be rotatable therewith; a multi-axis joint;
and two actuators coupled between the rotational member and
the antenna, or a structure rigidly coupled to the antenna,
characterised by an azimuth drive linkage, arranged to
transfer rotational motion from said rotatable member to
said antenna; said linkage including at least two pin joints
each rotatable about one of two mutually perpendicular axes,
one pin joint of which couples the azimuth drive linkage to
said rotatable member, and the other pin joint couples the
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azimuth drive linkage to the antenna; the weight of the
antenna (and any structure rigidly coupled to it) being
substantially wholly transmitted to the rotational
member on which it is mounted via the multi-axis joint
which is adapted to permit rotational movement about at
least said two mutually perpendicular axes; the two
actuators being controllable to stabilise the azimuth axis
of the antenna against pitch and roll motions of the ship,
and being positioned so that when they act in the same
sense they cause movement of the antenna about a ~irst
predetermined axis which is perpendicular to a second
predetermined axis about which the antenna is caused to
move when the two actuators act in mutually opposite senses.
Preferably two actuators are provided and which are
positioned so that each is ahle to turn the antenna about
one of two mutually perpendicular elevation axes. Preferably
the two actuators are positioned so that when they act in
the same sense they cause movement of the antenna about an
axis which is perpendicular to that axis about which the
antenna is caused to move when the two actuators act in
mutually opposite senses. These two axes are subsequently
referred to as the main elevation axis and the cross
elevation axis.
Preferably again each actuator is constituted by an elon-
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gate device whose Pfectlve length can be controllably
altered.
Conveniently the welght of the antenna (and any struc-
ture rigidly couDled to it~ ls transmltted to the rotational
member on which lt is mounted by means of a ball joint.
Preferably the antenna is constrained to rotate about th~
main elevation axis by two pin joints, which are both axially
aligned with the centre of rotation of the ball joint.
Preferably again the antenna is arranged to rotate about
the cross elevation axis by means of a third pin joint which
¦ is aligned with the cross elevation axis which passes through
¦ the centre of rotation of the ball joint. Conveniently, the
third pln joint is linked to ~he first two pin joints by
) means of a rigid linkage.
¦ 15 The invention is further described by way of example with
reference to the accompanying drawlngs, which show a stabillsed
antenna in accordance with the present invention,
¦ Figure 1 showing a part isometric vlew, and
¦ Figures 2 and 3 showing elevation and plan views res-
¦ 20 pectively
¦ Referring to the drawings, an antenna 20 is flxed rigidly
¦ ' to an antenna backi~g structure 1, which consists of a tubular
frame. The tubular frame is arranged so as to ~'a'lntain the
reflecting surface of the antenna rigidly in a predetermined
profile, since it is this profile whlch determines the
directional pro~erties'of the antenna. The structure 1 i5
mounted on a rotatable column 3, via a load carrying ball joint
2. The column 3 rotates about an axis relative to a fixed
support 10, which in turn is mounted rigidly on a ship. The
axis 11 about which the column 3 rotates is termed a training
axis. This axis'moves with the ship as it pitches and rolls,
¦ and is truly vertical only when the ship is perfectly still on
¦ placid water, and under this condition, the training axis 11
' coincides with the azimuth axis 12. The azlmuth axis 12 is
~5 the axis about whlch the antenna is arran~ed to rotate and is
constrained ~o be vertical by means of actuators 5, which link
i the column 3 ~o ~he structure 1. The actuators 5 consist of
~ elonyate members whos~ e~fective 'length can be rapldly and
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prec1sely adjusted by controllable adjustors 21 so AS to com~
pensate for the pitching and rollin~ motion of the ship.
The rotational motion of the column 3 is transmitted to
the structure 1 via a V-shaped linkage 4, which ls provlded
S with three pln joints 6, 7 and 8. Pin jolnts 7 and 8 lle on
the main elevatlon axis 23 which passes through the centre
of rotation of the ball joint 2, whereas the thlrd pin joint 6
is aligned with the cross elevation axls 22. The cross ele~
vation axis 22 also passes through the centre of rotation
Of the ball joint 2. The main elevation axis 23 and the cross
elevation axis 22 are arranged at right angles to each other.
Movement sensors are incorporated in the pin joints 6,
7 and 8 and the signals derived by these sensors are coupled to
the adjustors 21, so as to modify the ef~ective lengths of the
actuators 5, and thereby compensate for the ?itch and roll
movements of the ship on which the antenna is mounted. If it
is assumed that the longitudinal axis of the ship is parallel
with the cro~s elevation axis 22, then simultaneous operation
of the two actuators 5 in the same sense will compensate for
pitching motion of the ship, whereas simultaneous operation
of the two actuators 5 in a mutually opposite sense will com-
pensate for rolling motion of the ship. As theantenna rotates
in azimuth about the vert~cal axis 12, the actuators 5 are con-
tinually adjusted in length so as to compensate for these
motions.
Even if the axis 11 remains fixed but off-set from the
vertical axls lZ, it will be necessary for both actuators 5 to
alter their length as the antenna 20 completes each revolution
in azimuth. The rats at whlch the actuators 5 must operate in
this case is, of course, determined by the speed of revolution
of the antenna 201 In practice this speed of revolution may be
low compared to pitching and rolling movements which a ship
might experience in rough weather and it is necessary to ensure
that the actuators 5 are capable of sufficiently ra~id response.
It will be noted, particularly from the plan view shown
in Figure 3 that the actuators 5 are each orientated at 45
relative to the main elevatlon axis 23 and the cross ele~ation
j axis 22~ It is because of this orientation that ad~ustment
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ln the same sense of both actu~tors compensa-tes for pitchlng
motlon of the ship, whereas operatlon of both actuators in the
mutually opposite sense com~ensates or rolling mo~ion of the
ship.
The actuators S are each located at a node point 24 of the
structure 1. As will be apparent from the drawings, the structure
1 is of a ri~id tubular nature, and the node polnts 24, at
which a number of individual tubular members joln, provide
particularly strong attachment points. Addltionally, the node
points 24 are spaced apart from the surface of the antenna 20,
so as to enable the actuators 5 to obtain considerable lever-
age. This can be a very important consideration particularly when
strong gales are blowing a great deal of force ls reguired ln
order to controllably ori~ntate the antenna. The column 3,
which rotates relative to the fixed support 10, is a
relatively robust and rlgid structure and thelower ends of
the actuators 5 are mounted very closely adjacent to the reglon
at which it is most strongly supported by the upper end of
the fixed support 10. However, since the column 3 is not itself
stabllised, it is only the relatively light structure 1 com-
posed of tubular members and the thin sklnned antenna 20 which
~ are stabilised by the action of the actuators 5. This enables
the centre of gravity of the antenna arrangement as a whole
to be kept very low and close to the position of the fixed
support 10. The relatively light weight of the stabilised
portions of the antenna also enable a particularly rapid
response to unpredictable rollin~ and pitching motions of the
shi~.
Figure 3 also illustrates the way in which th~ feed horn
25 is mounted in front of the reflecting surface o the antenna
20 by a rigid but light framework 26. Electromagnetic energy
is coupled to the feed horn 25 via a waveguid~ 27. The wave-
guide 27 ls carried by one of the members of the tubular
framework ~6 and is coupled to a further waveguide portlon 28,
which is connected to the top of the column 3. It will be
noted that the waveguide 28 enters the column3 at a point
coincident with the axis 11, so that a simple concentric
rota~iny joint 30 enables the waveguide to pass ~rom the column
3 to the fixed suppork 10.
